The UK based, right-wing anti-immigrant think tank, suggests that full membership of the single market would mean remaining a fully signed up member to the freedom of movement principle.
It's paper, 'Immigration and UK Membership of the European Single Market,' concludes that net migration from the EU would be unlikely to fall below 155,000 if the UK remained a member of the single market — and despite the recent devaluation of the sterling, the wage disparity between the UK and Eastern Europe "will continue" to attract migration from poorer European countries.
"A reduction in the birth rate in Eastern Europe and the consequent decline in the population is likely to be counterbalanced by the attraction of a growing diaspora in Britain," the report states.
The report also warns that youth unemployment in countries including Greece and Italy where almost a quarter of young people are jobless, will continue to fuel migration to the UK.
Unemployment rates— chris g (@chrisg0000) December 14, 2016
Rep. Ireland 7.3%
EU ave (inc UK) 8.3%
"Under free movement these levels are very likely to continue for a considerable time.
"Such a population increase, would mean building the equivalent of a city the size of Birmingham every two years," the report adds.
"Adding 12.2 million to the UK population in just 25 years would be to increase the UK population by more than the equivalent of the entire populations of Bulgaria and New Zealand," it says.
Bulgaria and New Zealand have populations of 7.2 million and 4.6 million respectively.
Biggest source of extra EU migration is after 2012 €zone crisis from original members of EU15 — nearly doubles… then Romania, Bulgaria 2/5 pic.twitter.com/qPxhFVvDih— Faisal Islam (@faisalislam) December 1, 2016
"This research spells out the very serious consequences for our society of net migration continuing at its present scale with membership of the single market resulting in a relentless increase in our population," Alp Mehmet, vice chair of Migration Watch UK said.
"An increase of anything like 12 million in just 25 years is, quite simply, unacceptable to the British public and certainly not what they voted for in the referendum," he added.
However, leader of the Liberal Democrats, Tim Farron, hit back at Migration Watch's findings and said it was "no surprise that UKIP's [UK Independence Party] favorite think tank is pushing for a hard Brexit that would rip Britain out of the single market, costing jobs and risking the livelihoods of thousands of British people."
Mr. Farron suggested that the majority of people in Britain want to remain in the single market. A sentiment that chimes with the majority of big food and retail businesses, many of whom signed a letter to British broadsheet, The Times, warning that a hard Brexit would put British food supplies at risk.
"For our sector maintaining tariff-free access to the EU single market is a vital priority… It is where 75 percent of our food exports go, so all our farming and food businesses with to achieve this outcome," the letter stated.
President of the European Union Council, Donald Tusk recently clarified the EU's status and stance on Britain's negotiations, saying there would be "no compromises" on key EU principles, including access to the single market — which must come with the free movement of people.